1. Technical Field
The present invention relates to a backlight device including a light guide plate allowing light which is emitted from a light source and incident on an end surface thereof to be output from a light output surface thereof, an optical sheet diffusing the light from the light output surface and outputting the diffused light, and a resin frame member having an opening corresponding to the light output surface of the light guide plate and securing the positional relationship between the light guide plate and the optical sheet; and also relates to a liquid crystal display apparatus including such a backlight device.
2. Description of the Related Art
A liquid crystal display apparatus needs to include a backlight device which emits light onto a rear surface of a liquid crystal panel for transmission through the liquid crystal panel, since the liquid crystal panel itself does not emit light. Backlight devices are roughly classified into direct type backlight devices and edge type backlight devices.
An edge type backlight device includes LEDs (Light Emitting Diodes) that are arranged in, for example, a line and act as a light source, a light guide plate allowing light which is emitted from the light source and incident on an end surface thereof to foe output from a light output surface thereof, and an optical sheet diffusing the light from the light guide plate and outputting the diffused light onto a rear surface of a liquid crystal panel.
The edge type backlight device also includes a plastic (resin) chassis (frame member) securing the positional relationship between the light guide plate and the optical sheet, and a metal backlight chassis accommodating the light source and the light guide plate.
Recently, the distance between a display surface and the light source has been shortened along with the decrease in the width of a frame of a liquid crystal display apparatus. Therefore, the light source needs to be covered with the plastic chassis so as not to be directly viewable from the display surface.
WO2011/093119 (hereinafter “Patent Document 1”) discloses an illumination device including a light guide plate that is located such that an end surface thereof faces a light source and guides light incident on the end surface thereof toward a light output surface, an optical member covering the light output surface of the light guide plate, a positioning member determining, in a planar direction, the position of the optical member with respect to the light guide plate, and an accommodation member accommodating the light source, the light guide plate and the optical member. The positioning member includes a positioning hole formed in the optical member and a positioning pin that is provided on the light guide plate and is fit into the positioning hole.
The illumination device disclosed in Patent Document 1 determines the position of an optical sheet (optical member) by directly stacking the optical sheet on the light guide plate and presses a plastic chassis (pressing member) on the optical sheet so that the optical sheet is not disengaged from the light guide plate.
The light guide plate absorbs heat generated by the LEDs (light source) and moisture in the air to expand and contract and thus is changed in the size. In a state where the optical sheet is directly stacked on the light guide plate, there occurs a problem that the deformation of the light guide plate is transferred to the optical sheet and thus the optical sheet is sagged and wrinkled, as a result of which, the display quality of the liquid crystal display apparatus is significantly declined. There is another problem that the heat radiant from the LEDs is transferred to the optical sheet via the light guide plate and thus only an end of the optical sheet (in the vicinity of the LEDs) has a high temperature and is wrinkled.
In order to avoid such a situation, it is conceivable to provide a plastic chassis between the light guide plate and the optical sheet to form a space therebetween, so that the optical sheet is not directly stacked on the light guide plate. However, in this case, an arrangement that the light guide plate is processed (e.g., a pin is provided on the light guide plate) so that the light guide plate is used to determine the position of the optical sheet cannot be performed. Therefore, the optical sheet cannot be secured. This causes a problem that when an external force such as a vibration or an impact is applied to the liquid crystal display apparatus, the optical sheet is flapped and damages the liquid crystal panel, which may decline the display quality.